A French couple was on Thursday found guilty of murdering their 21-year-old au-pair and then burning her body on a bonfire in their London garden in September last year.

Sabrina Kouider, 35, broke into tears after she and partner Ouissem Medouni, 40, were found guilty by a jury at London’s Old Bailey court of murdering French live-in nanny Sophie Lionnet.

“Only Kouider and Medouni know exactly how they killed Sophie but the prosecution was able to prove that she died as a result of purposeful and sustained violence, and not by accident,” said state prosecutor Aisling Hosein.

“They were both jointly involved and came up with a plan to try and destroy her body and escape responsibility for this horrendous crime,” she added.

The jurors deliberated for a week before unanimously convicting Kouider and ruling by a majority decision of 10 to two that her partner Medouni was guilty.

They were both jointly involved and came up with a plan to try and destroy her body and escape responsibility for this horrendous crime

Aisling Hosein, prosecutor

Fashion designer Kouider is to undergo psychiatric assessment before sentencing, which is due to take place on June 26.

Both denied murdering Lionnet, from Troyes in eastern France, although they had admitted during the two-month trial to burning her body.

The court heard how the couple had interrogated and tortured Lionnet over their belief she was conspiring with one of Kouider’s former boyfriends – Mark Walton, a former member of Irish band Boyzone – who they claimed sexually abused members of their family.

Kouider, who has two children, told London’s Old Bailey court that she hit the au pair “really bad” with an electrical cable.

The victim’s mother called the guilty pair “monsters” in a statement read to court.

Catherine Devallonne said she “fell into shock and was hospitalised” after police broke the news that her daughter, whom she described as a “reserved young girl”, had been killed.

“I’ve been living this nightmare ever since,” she added, explaining that the victim’s brother was suffering from “mental trauma”, and had been placed with a foster family.

“Those monsters beat her to death,” she said. “They left her hungry. They took away her dignity and eventually her life.”

Police described how the victim was subjected to a “series of ‘interrogations’ … over a 12-day period, in a bid to force Sophie to confess to various false crimes they had accused her of.”

Kouider filmed some of the sessions, which they planned to hand them over to police as evidence of the nanny’s guilt.

“We will never know the full extent of the horrors Sophie had to endure,” said Scotland Yard detective Domenica Catino.

“Even in death, the torture, abuse and humiliation continued by placing her partially clothed body into a suitcase with no regard for even a semblance of a burial.

“It was clear that together the couple made the decision to torture Sophie and then in a cowardly fashion blamed each other for her death,” added the detective.

Firefighters discovered Medouni trying to burn Lionnet’s body on September 20 at the couple’s home in Southfields, south west London.

Neighbours alerted the authorities after noticing smoke and a “horrible” smell coming from the property.

Firefighter Thomas Hunt told the court that he confronted Medouni after he found human fingers and a nose as he put out the fire.

“I turned to the occupant and asked ‘Why are you burning a body?’”

“He said ‘It’s a sheep’,” Hunt added.

Kouider said in court that her partner – whom she called “Sam” – was violent to her and “was with me for sex, nothing else, and money”.